Automatic frequency control system for controlling the frequency of the local oscillator used in a differential phase modulated pcm receiver



-s. ExMlLL ER AUTOMATIC FREQUENCY CONTROL SYSTEM FOR CONTROLLING TH Nov.18, 1969 3,479,600

E FREQUENCY OF THE LOCAL OSCILLATOR USED IN A DIFFERENTIAL PHASEMODULATED PCM RECEIVER 3 Sheets-Sheet 1 Filed Oct. 5, 1966 kbQkbO ATTORNEV Nov. 18, 1969 s. E. MILLER 3,479,600

AUTOMATIC FREQUENCY CONTROL SYSTEM FOR CONTROLLING THE FREQUENCY FiledOct. 5. 1966 AMPLITUDE AMPLITUDE AMPLITUDE OF THE LOCAL OSCILLATOR USEDIN A DIFFERENTIAL PHASE MODULATED POM RECEIVER 3 Sheets-Sheet F/G. 3A

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FIG. 3B CYCLE-S FOR POS/T/VE DEV/A 7'/ON TIME F IG. 3C CYCLES FOR NEGA7'/VE DEV/,4 r/o/v I FIG. 4A

4 ERROR ANGLE 0c ERROR SIGNAL //v SUM BRANCH ERROR ANGLE cc 3' FIG. 4B

OUTPUT IN DIFFERENCE BRANCH 2' Nov. 18, 1969 5,-5, NHLLER 3,479,600

AUTOMATIC FREQUENCY CONTROL SYSTEM FOR CONTROLLING THE FREQUENCY OF THELOCAL OSCILLATOR USED IN A DIFFERENTIAL PHASE MODULATED POM RECEIVERFiled Oct. 5, 1966 3 Sheets-Sheet 5 OSC/LLATOR CORRECT/0N 5 7'0 VOLTAGE,ERRO 5mm; PHASE $NSOR D/RECT/ONAL [b COUPLER 23 \D/FFERENCE L I B 57NETWORK 0 a I J I BRANCH 2' 5 0r I I h :5' BRANCH 25- DETECTOR d l a 5/I J 4 Q J A 7'0 55 D/RECT/ONAL COUPLER 22 OUTPUT /9 I l l I I I IID/FFERENCE NETWORK I cofie/ws'cr/olv Z VOLTAGE I 7/ I I II HVBR/D Ilodb I I I 05x I72 SUM a/-| l I NETWORK IFFERENCE L \I/ETW0RK UnitedStates Patent US. Cl. 325423 3 Claims ABSTRACT OF THE DISCLOSURE Anautomatic frequency control system for use in a differential phasemodulated PCM signal'receiver in which an error signal produced in thesum branch and/or the difference branch of a differential phase detectoris used to generate a correction voltage for the receiver localoscillator. The AFC circuit includes means for determining the phase ofthe error signal and means for minimizing the effects of informationcontent upon the phase of the correction voltage.

This invention relates to automatic frequency control (AFC) arrangementsfor controlling the frequency of the local oscillator used inFM-differential phase shift PCM communications systems. Moreparticularly, it relates to AFC arangements which will operate for anyarbitrary PCM pulse pattern.

In the copending application by W. D. Warters, Ser. No. 568,893, filedJuly 29, 1966, there is described a differential phase PCM system whichutilizes frequency modulation techniques to produce the required phasemodulation (FM-DPM). It is readily apparent that for optimum operationof such a system, the inadvertent introduction of spurious frequencymodulation is advantageously avoided. Since one possible source ofsignificant spurious modulation is the local oscillator used todownconvert the carrier signal at repeater stations or at the receiverend of the system, it is advantageous to provide some means forautomatically controlling the local oscillator frequency.

In accordance with the present invention, the error signal produced inthe sum branch and/or the difference branch of a differential phasedetector (of the type used in the receiver of a differential phasemodulated PCM communications systems) is used to generate the oscillatorcorrection voltage for the receiver local oscillator. Means are providedfor determining the phase of the error signal in order to applyfrequency correction in the proper sense. Means are also provided forminimizing the effects of information content upon the phase of thecorrection voltage.

In one specific embodiment of the invention, the error signal in the sumbranch of the differential phase detector is used to generate theoscillator correction voltage. In a second embodiment of the invention,the error signal in both the sum and the difference branches of thedifferential phase detector are employed.

These and other objects and advantages, the nature of the presentinvention, and its various features, will appear more fully uponconsideration of the various illustrative embodiments now to bedescribed in detail in connection with the accompanying drawings, inwhich:

FIG. 1 shows an FM-DPM receiver including a differential phase detectorand an automatic frequency control circuit in accordance with theinvention;

FIG. 2, included for purposes of explanation, shows the frequencydeviation for an illustrative signal pulse train;

FIGS. 3A, 3B and 3C, included for purposes of explanation, show thenumber of intermediate frequency cycles per pulse time interval fordifferent signal conditions;

FIGS. 4A and 4B, included for purposes of explanation, are vectordiagrams showing the signal components in the sum and differencebranches of the differential phase detector when the local oscillator isimproperly tuned;

FIG. 5 is an illustrative embodiment of a phase sensor for use in an AFCcircuit in accordance with the invention; and

FIG. 6 shows an AFC circuit that is independent of the informationcontent of the signal.

Referring to the drawings, FIG. 1 shows, in block diagram, a portion ofa receiver for use in a FM-DPM transimssion system, including adown-converter 10, a voltage-controlled local oscillator 11, and adifferential phase detector 12. The latter, whose mode of operation iswell known in the art, and is described in some detail in theabove-cited copending application by W. D. Warters, typically comprisesa pair of similar hybrid junctions 13 and 14, each of which has twopairs of conjugate branches. The pairs of conjugate branches associatedwith hybrid 13 are designated 1-2 and 3-4. Those associated with hybrid14 are designated 12' and 3'-4'.

As illustrated in FIG. 1, branch 1 of hybrid 13 is the input branch towhich the intermediate frequency signal is applied. Branch 2 isresistively terminated. Branches 3 and 4 of hybrid 13 are connected tobranches 3' and 4', respectively, of hybrid 14 by means of wavepaths 15and 16. One of the wavepaths 16 includes a delay network 17 for reasonswhich will be explained in greater detail hereinbelow.

The remaining branches 1' and 2' of hybrid 14 are connected respectivelyto detec ors 18 and 19 which, in turn, are connected to a resistiveoutput network 20. The phase detector output is obtained from thisnetwork.

It is the function of the phase detector to compare the relative phaseof the signals in adjacent time slots. In a binary system, a comparisonwhich indicates that there has been no relative phase shift isindicative of one of the two binary states, whereas a degree phase shiftis indicative of the other binary state. Accordingly, there are means 17provided in wavepath 16 to delay the signal in path 16 one time slot andto introduce'anyadditional delay which may be needed to establishthedesired inphase or out-of-phase signal relationships'at branches 3'and 4' of hybrid 14. If the two signal componen s from adjacent timeslots are in phase, they combine in branch 1', (designated the sumbranch) causing a current to flow in the detector 18 and in theresistive network 20 in a direction to produce a positive output pulse.If, on the other hand, the two signal components are out of phase, theycombine in branch 2' (designated the difference branch) thereby causinga current to flow in the detector 19 and in resistive network 20 in theopposite direction, thus producing a negative output pulse. In thismanner the original binary baseband signal is recovered.

It is readily apparent that if there is any drift in the frequency ofthe local oscillator a spurious phase indication can be obtained, and anerror introduced in the detected signal. To minimize this possibility,an automatic frequency control (AFC) circuit 21 is included in thereceiver to control the frequency of local oscillator 11. The AFCcircuit, which will be described in greater detail hereinbelow, samplesthe signals in the sum difference branches 1' and 2' of hybrid 14 bymeans of directional couplers 22 and 23, and generates an oscillatorcorrec- 3 4 tion voltage in a phase sensor 25 to correct the local 14 tocorrect the local oscillator frequency. The phase oscillator frequency.sensor circuit, which senses the phase of the error signal,

The operation of the AFC circuit can be explained by comprises a 180degree hybrid junction 50, having two considering the specific signalpulse train illustrated in pairs of conjugate branches 51-52 and 53-54.Branch FIG. 2, which includes three pulses during which the 5 51 isconnected'to one of the directional couplers 22 signal carrier frequencyincreases, followed by two pulses through a delay network 55. Branch 52is connected to during which the Signal carrier frequency decreases. Thethe other directional coupler 23. The other two branches pulse duration1- and the sampling periods 0, 1, 2 et cetera 53 and 54 are connected toamplitude detectors 59 and are also indicated in FIG. 2. For purposes ofillustration, 56, respectively The output from the detectors are, in anintermediate frequency f is selected such that there turn, connected toa difference network 57, which genare 4% cycles in the interval 1Therefore, crates the oscillator correction voltage.

In operation, directional couplers 22 and 23 sample (1) the signals inthe sum and difference branches 1' and 2', respectively, of hybrid 14,and couple the sampled signals Because of the frequency modulation, thephase of the t0 the Sense! eii'euityP Y 3, to 10 db Couplers can signalis either advanced or retarded by 90 degrees durhe used for this p p ASindicated in F GS- 4A a d ing each pulse period. This means that duringthe first the Sampled signals are approximately in time quadthree timeintervals of the signal depicted in FIG. 2, when ratilie, and are 50indicated y the two Vectors a nd b the deviation is positive, there are4% (distorted) I.F. at the terminals of branches A and resPeetiveiys ofcycles, h re during th l t t ti i l h the sensor circuit. To establishthe desired in-phase and the deviation is negative, there are only 4 LF.cycles. 'P l'eiatiehships required at hybrid ill the This is illustratedin FIGS. 3A, 3B and 3c which show, sensor network, a quarter wavelengthdelay 5 i i t respectively, 4% cycles in time period 1 for theunmodudlleed in branch A which further delays Signal a 50 that latedcarrier signal, 4 /2 cycles for a positive frequency the Signals a andi? are 180 degrees out Of Phase at deviation, and 4 cycles for anegative frequency deviation. hie-riches 51 and 52 of hybrid Haif 0teach Signal a When the signal in adjacent time slots are compared at bip to branch 54 0f the hybrid With no times 0, 1, 2 et cetera, it isseen that they are out of phase further feietive Phase Shift, and out ofPhase The h Af i i i whereas h are i h h M other two halves of signals aand b couple to branch 53 is negative. of the hybrid with an additional180 degree phase shift,

By referring to the curves shown in FIGS. 3B and 3C, and add in P thefollowing tabulation of normalized currents (or volt- The tWO resultantSignals derived f om hybrid 50 are ages) at the various branches ofhybrid junction 14 can mpli -detected in detectors 56 and 59, and thedifferbe made for the pulse sequence illustrated in FIG. 2. ence intheir amplitudes derived in the difference net- TABLE I Sampling time 01 2 3 4 5 6 Location:

4' 0. 707 1r 0. 707 L0 0. 707 l 7r 0. 707 L0 0. 707 0 0. 707 0 0. 707 7r2 (difference branch) 1. 0 L 0 l l 7r 1l 0 0 0 1 7r 0 From thistabulation it is seen that when the signal work 57. This differencesignal is the oscillator correction components in branches 3' and 4 areequal, and out of voltage. phase, as they are at sampling periods 0, l,2 and 5 (here- As indicated above, it is the function of the phaseinafter known as a time 0 type signal), they combine sensor 25 todetermine the sense of the error signal genin the difference branch 2,with no signal being coupled erated in the differential phase detector.In this connecto the sum branch 1'. Similarly, when the signals in tion,it can readily be shown that if a negative error angle branches 3 and 4'are equal, and in phase, as they are is assumed in FIG. 4A, signalcomponent a is reversed at sampling times 3, 4 and 6 (hereinafter knownas a 180 degrees, and as a result a difference signal of opposite time 3type signal), they combine in the sum branch sign is produced in thedifference network 57. Thus, the 1', with no signal being coupled tothe, difference branch sense Of the correction voltage is determined bythe sense 2'. If, however, local oscillator 11 is not properly tuned, Ofthe error. the two signal components will not be exactly out of Theoperation described hereinabove, is based upon phase, or exactly inphase. Instead there is an error angle pulse conditions at samplingtimes 0, 2 d 5- If t introduced between the two signal components whichoperation of the AFC circuit is also examined at times produces an errorsignal when the signal components 4 and at which times the error Signalpp in the are subtracted from each other. This is illustrated in thedifference branch the n of the oscillator rr c i n vector diagram inFIG. 4A which shows the two signal voltage is found to be reversed fromthat obtained at components in branches 3' and 4' for a time 0 typesampl g times 2 and s eans t at t e sign signal, and the resulting errorsignal produced in the sum of the Correction Voltage is not independentof the inforbranch 1'. FIG. 4B shows the two signal components in 60mation content of the signal. It thus becomes necessary to branches 3'and 4, and how they combine in the din bias the system in favor of oneor the other of the two ence branch 2', signal conditions and to adjustthe system accordingly.

It will be noted in FIG. 4A that the resultant signal This can be done,for x p y attenuating the signal in the sum branch has a phase whichdepends upon the b 50 that When the error Signal occurs in branch itSsign of the error signal it. If the local oscillator frequencycontribution is attenuated. It can then be shown that for drifts in theopposite direction from that illustrated, the random Signal content, theP a COIIditiOnS a l s error angle is negative, and the direction of theerror y a time YP Signal Will dominate the AFC y signal in the sumbranch reverses its direction by approxiand P p control of theOsieiietor frequency will be mately 180 degrees. This signal cantherefore be used as maintaineda means for generating the AFC correctionvoltage since e method of biasing the AFC System is to design its phasevaries as a function of the sense of the local directional eeupiel' 23Such that its coefficient of Coupling oscillator frequency error, isless than the coefficient of coupling of directional FIG. 5 is anillustrative embodiment of the phase Coupler 22 y approximately 10deeiheis- Aitemetiveiy, sensor circuit, indicated as block 25 in FIG. 1,for utilizthe directional couplers can be the same, and an ating theerror signal produced in the sum branch of hybrid tenuator added tobranch B of the AFC circuit, or any other circuit arrangement can bemade such that the overall means whereby the sum and difference branchesof the phase detector are sampled are sufficiently unequal that littleor no correction voltage is generated when the error signal occurs inbranch 2 of hybrid 14.

While an error signal generated in branch 2' of hybrid 14 causes anincorrectly phased correction signal to be generated in the phase sensorcircuit 25, there is nevertheless no reason why this error signal cannotbe used to generate a properly phased correction signal. This can bedone simply by providing a second phase sensor, adapted to operate withan error signal derived from difference branch 2. This second sensor canbe made to develop a second correction signal having the correct sense,which can then be added to the correction signal generated by an errorsignal in branch 1'. Such an arrangement is shown in FIG. 6, whichutilizes two phase sensor circuits 80 and 81 in which the rolls playedby the w and b signals are reversed. Specifically, phase sensor 81operates upon time 3 type signals, whereas phase sensor 80 operates, asdescribed hereinbefore, on time 0 type signals. The correction voltagegenerated by both type signals are added in a sum network 72 to producethe local oscillator correction voltage. To minimize the effect of time3 type signals upon phase sensor 80, the b signal coupled to hybrid 70is attenuated relative to the a signal by means of a decibel directionalcoupler 73 located between the b signal circuit and hybrid 70.Similarly, to minimize the effect of tim 0 type signals upon phasesensor 81, the a signal coupled to hybrid 71 is attenuated relative tothe b signal by means of a 10 decibel directional coupler 74 locatedbetween the a signal circuit and hybrid 71.

This latter arrangement, using two separate phase sensors, has theadvantage that a local oscillator correction voltage of the proper senseis generated regardless of the information content of the signal. On theother hand, if only one phase sensor is used in the AFC circuit, as inFIG. 1, there is always the possibility that the information content ofthe signal might be limited to a time 3 type signal over an extendedperiod of time and, as a result, no correction voltage would begenerated.

In all cases it is understood that the above described arrangements areillustrative of but a small number of the many possible specificembodiments which can represent applications of the principles of theinvention. Numerous and varied other arrangements can readily be devisedin accordance with these principles by those skilled in the art withoutdeparting from the spirit and scope of the invention.

What is claimed is:

1. In a differential phase modulated, pulse code modulation system, areceiver including:

a voltage-controlled oscillator and a differential phase detector;

said detector having sum and difference signal branches;

sampling means having unequal sensitivities for extracting a portion ofthe signal in each of said branches;

means for adding and for substracting components of said extractedsignal portions to produce a sum signal and a difference signal;

means for amplitude detecting said sum and said dif- -ference signals;

means for substracting said detected signals; and

means for applying the resultant voltage derived from said subtractingmeans to said local oscillator for controlling the frequency thereof.

2. Automatic means for controlling the frequency of a local oscillatorin a differential phase modulated PCM signal receiver comprising:

a frequency down-converter;

a voltage-controlled oscillator;

a differential phase detector having sum and difference signal branches;

means for coupling said oscillator to said down-converter;

means for coupling said down-converter to said differential phasedetector;

first and second phase sensor circuits;

means for coupling a fraction of the signal in said sum branch to saidfirst phase sensor;

means for coupling a fraction of the signal in said difference branch tosaid second phase sensor;

means for coupling a smaller fraction of the signal in said sum branchto said second phase sensor;

means for coupling a smaller fraction of the signal in said differencebranch to said first phas sensor;

characterized in that said phase sensors produce, in response to saidcoupled signals, an error correction voltage whose amplitude and phaseis a function of the detuning of said oscillator;

a sum network for adding said error correction voltages; and

means for applying the resultant voltage derived from sum network tosaid oscillator for correcting the frequency thereof.

3. Th automatic frequency control circuit according to claim 2 whereineach phase sensor includes:

means for adding and means for substracting components of said coupledsignals to produce sum and difference signals;

means for amplitude detecting said sum and difference signals; and

means for substracting said detected signals.

References Cited UNITED STATES PATENTS 3,181,122 4/1965 Brown 340KATHLEEN H. CLAFFY, Primary Examiner C. JIRAUCH, Assistant Examiner US.Cl. X.R. 325-346

